Also, to add to Wayne's answer, there are multiple antennas because the computer that processes the data received compares the phase of the signals received from each of them to calculate the direction the signal came from... Because they are separated from one another, any signal received from a point directly in front of them will be received at the same time (it travels the same distance), and will therefore be in phase with the signals from the others.
But if the source is off to one side, then the signal received from the near antenna will have traveled a shorter distance, and will be out of phase with the others by an amount proportional to the sine of the angle to the source.